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In this post we try to diagnose a burnt SMPS circuit and try to troubleshoot and repair the circuit. The shown unit is a cheap readymade Chinese make SMPS circuit. This article is written as per the request made by Mr. Kesava.

My SMPS got Burnt

Dear sir,You help me lot sir...The below attachment is 12v 1.3 amps SMPS for charging Agriculture Sprayer..If charge full the green led will glow...If charge low the red led will glow...

But now this charge not working...And i check inside , The AC input bridge rectifier IN4007 1diode got damaged...i replace it with new one diode..Now the new diode also damaged....Pls guide me sir....

In our area shop..this type of chargers are not available sir...But my aim is not to buy new one..i itself want to rectify with u r guidance sir....Pls help me sir....

Sorry for bad english.I'm not good sir...

Thanks & Regards N.Kesavaraj

Troubleshooting the Problem

Hi Kesava,

It's most probably due to a burnt mosfet, the one which can be seen on a heatsink. You can try replacing it with a new one, and also make sure to change the adjoining 10 ohm resistor which also looks like it is burnt.

Regards.

Repairing the SMPS Circuit

Referring to the images above, the primary side of the unit appears to be the popular 1 amp 12V SMPS adaptor using a mosfet based switching design, and includes an opamp based auto cut off charger section at its secondary section of the board

From the first two images we can clearly see that one of the diodes being completely blown apart, and responsible for shutting down the entire circuit board.

A bridge rectifier can be normally seen at the beginning of any SMPS circuit and is introduced primarily for rectifying the mains AC to a full wave DC, which is further filtered using a filter capacitor and applied to the mosfet/inductor stage for the intended primary side switching operation.

This primary side switching causes an equivalent low voltage pulsating DC to be induced at the secondary side of the transformer, which is then smoothed using a large value filter capacitor at the the secondary side for acquiring the final stepped down SMPS DC output.

The diodes in the bridge rectifier appears to be the normal 1N4007 diodes which are capable of handling not more than 1 amp current, therefore if this 1 amp value exceeds the diodes can get ripped through and damaged.

The diode might have burnt due to a high current passage which in turn might have happened due to a stalled mofet inductor operation. Which means that the mosfet might have stopped osculating causing a short circuit through itself, allowing the entire AC to go through the components within the input supply line.

How to Repair the SMPS Circuit.

2) Without any doubt you will find the mosfet being the faulty component, so you can quickly go for a replacement of the same using a correctly matched mosfet

3) After changing the mosfet make sure to change the burnt rectifier diode also, and ideally change all the 4 diodes in the bridge, to ensure no weakened diodes are present in the network.

4) You may also want to check if there are any other parts such as resistors or thermistor that may look suspicious and if any replace them with new ones.

5) Once all the doubtful elements are replaced it is time to switch ON the SMPS for the final verification.

However this must be done with a series protection load in the form of a series incandescent bulb to make sure that the circuit does not blow of due to some other hidden fault. A 25 watt bulb will be just good for safeguarding the unit from any catastrophic circumstances.

6) On switching on the SMPS, if the the bulb does not glow, it would probably indicate all's well and the unit has been repaired successfully. Now you can feel free to check the output voltage of the SMPS with a meter and confirm that it is producing the right readings.

7) Finally without removing the bulb connect an appropriately rated DC load and check whether it is working correctly or not.

8) If everything seems to be working normally you can remove the series bulb, and repeat the testing process, but make sure to include a small fuse in series with the input supply permanently.

9) However in case the bulb shows a bright glow, would indicate a serious problem persisting in the SMPS circuit and will need to be investigated afresh, this may be done by first switching off the unit and then checking each and every component in the primary side of the trafanformer.

10) The components which needing a recheck will be fundamentally the ones which are prone to high voltage and current damage, such as small BJTs, diodes and low value resistors.

11) The components which can be left unchecked are the ones which are adequately rated and are able to protect itself from from high voltage and current inrush. These may include high value resistors above 50K, or low value wirewound resistors above 1K.

Similarly, capacitors which may be rated above 200V can be left unchecked unless one of these look somewhat damaged externally.

Testing for a Burnt Inductor Transformer

Every SMPS circuit will essentially include a small ferrite transformer, which this part can also possibly become the cause of a burnt SMPS circuit, although the chances of a damaged transformer can be too remote.

This is because the wires inside the inductor might require some time to burn, and before this can transpire the other more vulnerable parts such as diodes and transistors would be forced to blow off ,preventing any further damage to the inductor.

So basically you can be rest assured that the transformer is the one element which might be the safest and the undamaged part in a given faulty SMPS circuit.

If in a rare event the inductor burns, this would be distinctly visible from the burnt insulation tape which may be also melted and stuck with the winding. An SMPS with a burnt transformer could be virtually irreparable, because a burnt transformer would mean most of the elements burnt out, along with PCB tracks uprooted. Time to buy a new SMPS unit.

The secondary side mostly will not require any checking as it is isolated from the primary and can be expected to be aloof from the dangers.

Well, this concludes this article explaining tips to repair an SMPS circuit, if you think I have missed some crucial points, or if you have something important to add in the list, please tell us through your valuable comments.

In this post we learn how to apply the IC L6565 for making a compact multi purpose 110V, 14V, 5V SMPS circuit using minimum number of external components.

Implementing quasi-resonant ZVS flyback

The IC L6565 from ST Microelectronics is designed as a current-mode primary controller chip, to specifically suit quasi-resonant ZVS flyback converter applications. The quasi-resonant implementation is accomplished through demagnetization of a transformer sensing input, which is used for switching ON an attached power mosfet.

During the operations of this IC in a converter, the variations in the power capacity of the converter becomes compensated by a line feed forward stage acquired through the line voltage.

Circuit Schematic

Whenever the connected load is minimal or absent, the IC displays a unique feature which automatically brings down the operating frequency of the converter, and yet ensuring the operation at as far as possible around the ZVS level.

Converters using IC L6565 not only enables low consumption of the design through a low start up current, and a sustained low quiescent current, the system makes sure that it perfectly complies with Blue Angel and Energy Star SMPS guidelines.

In addition to the above explained features, the chip also includes an configurable auto disable function, an in-built current sense and shut down function, and also an accurate reference voltage input for executing basic regulation functions, and an ideal two stage overload protection.

How this 110V/14V/5V SMPS works:

In quasi-resonant SMPS circuits, the operation is implemented by synchronizing the mosfet's switch ON frequency with the demagnetization frequency of the transformer, which is generally accomplished by sensing the falling edge or the negative edge of the transformer's relevant winding voltage.

The above procedure is very simply executed by the IC L6565 through an exclusively designated pinout and using just a single resistor.

This operation enables the voltage, current variable frequency operation feature (in response to a varying input voltage current situations).

The circuit is designed to run approximately within the DCM (Discontinuous Conduction Mode) and CCM (Continuous Conduction Mode) operational mode of the transformer, which can be compared quite like a ringing self-oscillating choke converter or RCC converter.

The pin#8 which is the Vcc of the IC acquires an operating supply voltage from an external regulator network, which sets a 7V rail internally, and this voltage helps to run the entire functionality of the IC and for all the specified executions, associated with its remaining pinouts.

The IC includes an in-built bandgap circuit which enables the generation of an accurate 2.5V reference voltage for ensuring an improved regulation to the control loop used with primary feedback functionality.

You will also find an under-voltage lockout or UVLO comparator with hysteresis featured in the design, which allows the chip to shut down in case the Vcc drops below a specified voltage limit.

A zero current detection stage which is integrated within the IC becomes responsible or switching the external power mosfet in response to every negative edged pulse below the 1.6V level that's fed to this relevant pinout marked as ZCD (pin#5).

However keeping the noise immunity factor in mind and to control it effectively, the associated triggering block must get activated before pin#5 is allowed to fall below 1.6V, by enabling a +2.1V on this pinout.

This process helps the detection of the transformer demagnetization required for the quasi-resonanat operation, in which the transformer's auxiliary winding provides the required signal to the ZCD input, in addition to the IC supply.

In an alternate method where the mosfets may be intended to run in the PWM mode rather than quasi-resonanat mode, the above process can be employed for synchronizing the mosfet switch ON in response to negative pulses from an external source.

Shut Down Option

In such cases the triggering block is forced to go through momentary shut down as soon as the mosfet is switched OFF. This helps to achive a couple of objectives:

1) To ensure that the negative edged pulses following the leakage inductance demagnetization does not accidentally triggers the ZCD circuit stage, and
2) To acknowledge the functioning termed as frequency foldback.

To initiate the external mosfet at the start up, an internal starter stages i used, which enable the driver stage to execute a triggering pulse to the mosfet gate, this becomes necessary due to the absence of an initializing signal to the mosfet from the ZCD pin.

In order to keep the external component to minimum associated with auxiliary winding or a possible external clock, the voltage at the ZCD pin is enabled with a double clamping.

The upper clamp voltage is fixed at 5.2V while the lower clamping potential is rendered at one VBE over the ground level.

This enable the interface to be configured using a just one external resistor for limiting the sourced current which is furthermore shunted by the relevant pinout as per the specified values set for the internal clamping voltages.

For more info regarding the additional internal stages of this 110V, 14V and 5V rated SMPS circuit, you can refer to the original datasheet of L6565

In this post we comprehensively discuss a simple 12V 2 amp SMPS circuit using the IC UC2842.

Let’s try to understand the functions and criticalities of a few of the main components used in this 12V 2 amp SMPS circuit:

Cin Input Bulk capacitor and minimum bulk voltage:

The shown bulk capacitor Cin may be incorporated using a single or a few capacitors in parallel, possibly by using an inductor across them to eliminate noise generated due to differential-mode conduction. The value of this capacitor decides the level of minimum bulk voltage.

If a lower value Cin is used to reduce minimum bulk voltage, might result in raised primary peak current overloading the switching mosfets and also the transformer.

On the contrary keeping the value larger might result in higher peak current on the mosfet and the trafo, which is also not acceptable, therefore a reasonable value as indicated in the diagram should be chosen.
It may be done by using the following formula:

Here Vin(min) indicate the RMS value of the minimum AC input voltage which is around 85 V RMS.

fLINE(min)denotes the frequency of the above RMS value which may be assumed to be 47Hz.

With reference to the above equation, in order to achieve a minimum of 75V bulk voltage value, at 85% efficiency, the Cin value will need to be around 126uF, in our prototype 180uF was found to be just fine.

Calculating the Tansformer turn ratios:

To begin with the transformer turn calculation, the most favorable switching frequency needs to be found out.

Although the IC UC2842 is specified to produce a maximum frequency of 500kHz, considering all the feasible and efficiency related parameters it was decided to select and set the device at around 110kHz.

This allowed the design to be reasonably well balanced in terms of the transformer size, EMI filter dimension and still keep the operations within the tolerable losses.

The term Nps refers to the primary of the transformer and this may be determined depending upon the rating of the driver mosfet used along with the rating of the secondary rectifier diode specifications.

For an optimal mosfet rating we first need to calculate the peak bulk voltage with reference to the maximum RMS voltage value which is 265V input AC in our case. Therefore we have:

For the sake of simplicity and cost effectiveness, a 650V rated mosfet IRFB9N65A was selected for this 12V 2 amp smps circuit prototype.

If we consider the maximum voltage stress on the mosfet drain to be around 80% of its specifications, and taking 30% as the permissible voltage spike from the maximum bulk input supply, the resultant reflected output voltage can be expected to be lower than 130V as expressed in the following equation:

Therefore for a 12V output the maximum primary/secondary transformer turn ratio or the NPS may be calculated as indicated in the following equation:

In our design a turn ratio of Nps = 10 has been incorporated.

This winding must calculated in such a way that it is able to produce a voltage that may be a little higher than the minimum Vcc specification of the IC, so that the IC is able to operate under optimal conditions and stability is maintained throughout the circuit.

The auxiliary winding Npa can be calculated as shown in the following formula:

The auxiliary winding in the transformer is used for biasing and providing the operating supply to the IC.

Now for the output diode, the voltage stress on it may be equivalent to the output voltage and the reflected input supply, as given below:

In order to counter the voltage spikes due to “ringing” phenomenon, a Schottky diode rated with a blocking voltage of 60V or higher was felt necessary and employed in this design.

Also to keep away high voltage current spike factor, this flyback converter is designed to work with a continuos conduction mode (CCM).

Calculating the Maximum Duty Cycle:

As discussed in the above paragraph, once we calculate the NPS of the transformer, the required maximum duty cycle Dmax can be calculated through the transfer function as allocated for CCM based converters, the details can eb witnessed below:

Transformer Inductance and Peak Current

In our discussed 12V 2 amp smps circuit the transformer magnetizing inductance Lp was determined as per the CCM parameters. In this example the inductance was chosen such that the converter is able to get into the CCM working zone with around 10% load and using minimum bulk voltage in order to keep the output ripple to the lowest.

For more details regarding the various technical specifications and formulas you can study the original datasheet here

The presented 7 watt LED driver circuit is an SMPS based non-isolated, transformerless circuit which ensures a safe current controlled output for the attached LED, it is very affordable to build without involving complex transformer winding.

The aim behind the design of the IC TPS92310 (from TEXAS INSTRUMENTS) is to offer a constant current line and load regulation to the load through a primary side sensing flyback inductor, which operates in the critical conduction mode, and eliminates the need of the traditional opto coupler based secondary side feedback control.

The proposed design employs a non-isolated single inductor smps design and thus removes the obligatory transformers, making the design much compact and involving less BOM, yet meets the standard performance criteria of an LED driver specifications.

The following explanation provides us with the operating principle of the proposed 7 watt LED driver SMPS circuit:

1) The LED controller chip TPS92314A includes an advanced constant ON-time control feature for ensuring a high power factor at the input, and quasi-resonant switching for guarantying greater efficiency and minimum EMI emission.

2) The design facilitates load power regulation through the stored energy of an inductor configured in the form of a high-side buck converter .

3) The inclusion of a diode/capacitor at the output additionally regulates the DC content, without depending on any extra auxiliary winding which is commonly seen in traditional isolated forms of SMPS designs...here this is eliminated causing the unit to become very compact, highly efficient and cost effective.

4) The figure shows a standard full bridge rectifier network at the input for converting the alternating input current into a single positive AC bus.

The pulsating sine voltage here faithfully follows the pulsating sine current. due to the presence of a 100nF capacitor immediately after the bridge rectifier, and this helps in maintaining a high power factor response.

5) The above processed supply is fed to the drain of a mosfet which is configured as a high side switching device, having its source hooked up with D8 freewheeling diode along with inductor L3 and output capacitor C5.

6) In the figure the IC input side of the IC could be seen referenced to a switching junction SW, which makes sure that the IC does not switch ON until the processed AC has a potential higher than the connected LED's forward voltage value, and also for so along as the input is not drawing any current. This parameter causes a delay factor during power switch, and can be calculated through the following expression:

Δ T = Sine (inverse)VLED / √2 xVac

During the critical conduction mode periods of the IC TPS92314, the peak current from the inductor becomes two times more than the input peak current.

The inductor value for this 7 watt LED driver SMPS circuit can be calculated using the following formula:

L = [1.41 x Vac - VLED] x Ton / ΔIpeak

Due to the fact that this IC involves a critical conduction mode operation implies that the every subsequent ON periods is initiated only once the current within the inductor has ramped down to almost zero.

A feedback voltage in the form of VLED is applied back to the IC which acts like a supply voltage for the IC, because VLED can be seen linked with the input side bridge network ground. This particular implementation allows the design to comfortably work with only a single non-isolated inductor and gets rid of the complex extra biasing winding.

This makes this 7 watt non isolated SMPS LED driver circuit extremely compact, durable, efficient and very long lasting and also compliant with the present SMPS laws.

The design can be adapted for all power LEDs ranging from 1 watt to 7 watt.

The main specifications of the driver circuit can be witnessed in the following table:

So far in this website we have studied LM317 based linear power supply circuits, here we will learn how an LM317 can be executed as a variable switch mode power or SMPS with zero loss.

LM317 as Linear Regulator

We all know that an LM317 IC is internally designed to work as a linear voltage regulator IC, which has a serious drawback of power dissipation through heating. Moreover such topology also requires the input to be minimum 3V higher than the desired output, adding further restrictions to the given regulator configuration.

Here we discuss how the same IC could be simply implemented as a 0-40V variable power supply using SMPS topology and therefore eliminating the losses mentioned in the above paragraph.

Modifying LM317 Circuit into a PWM Switching Regulator Circuit

The LM317 variable SMPS circuit explained here effortlessly converts an ordinary LM317 IC into an inductor based switching regulator power supply counterpart, as exhibited in the following diagram:

Circuit Schematic

Referring to the above shown diagram we can see that the LM317 is configured in its usual variable regulator mode but with some additional parts in the form of R6, C3, and D1.

We can also see an inductor attached with D1 and an associated power BJT Q1.

How the Circuit Works

Here the LM317 IC performs two tasks together. It varies the output voltage through the indicated pot R4, and in turn causes a PWM to initiate for the base of Q1.

Basically, the introduction of R6/C3 transforms the LM317 regulator circuit into a high frequency oscillator circuit, forcing the output of the LM317 to switch ON/OFF rapidly with a varying PWM, which is dependent on the setting of R4.

The BJT Q1 along with the inductor L1 and D1 forms a standard buck converter circuit which is controlled by the above explained PWM generated by the LM317 circuit.

This implies that while the pot R4 is varied, the voltage pulse width developed across R1 also varies proportionately causing Q1 to switch L1 in accordance with the varying PWMs.

Capacitor C4 makes sure that the fluctuating output from L1 at the output is adequately smoothed and eliminated, this consequently enhances the ripple current into a stable DC.

In the proposed LM317 switch mode power supply circuit since the IC LM317 is not directly involved with the handling of the load current, it's restricted from dissipating current, and thus ensures an efficient regulation of the high input voltage into the desired low output voltage levels.

The design also allows the user to upgrade the circuit into a high current SMPS circuit simply by changing the Q1, L1, D1 rating as per the required output current specifications.

Here we learn about a simple switch mode power supply (SMPS) circuit which is capable of delivering 3.3V, 5V, 9V at around 800mA from a Mains input range of 100V to 285V, with a complete isolation.

The functioning of the proposed 3.3V, 5V, 9V SMPS Circuit may be understood by going through the following detailed circuit explanation:

Input EMI Filtering

The capacitor C10, together with C13 form the main capacitors. In conjunction with L4, they constitute the EMI filter stage.

Tiny Switch's integrated frequency jitter facilitates to accomplish a reduced EMI even with a rather uncomplicated EMI filter configuration.

Primary Clamp Snubber Stage

D3, R1, R2, along with C1 build up the primary side clamp-snubber to fix the voltage peak at the Drain pin as soon as its turned-off. D3 is a 1N4007G, a glass-passivated model of the typical 1N4007, with regulated back EMF restoration. Its positioned, in conjunction with R2, to enhance EMI and the intended 3.3V, 5V, 9V outputs.

In a situation where a specific fast recovery diode is difficult to obtain, any other alternative fast diode can be incorporated without an issue.

Output voltage filtering

C3, C5, and C7 become responsible as the bulk output capacitors. C4, C6, and C8, in conjunction with the inductors L1, L2, and L3, constitute second-stage output filters.

Output Feedback Loop

Sensing resistors R4 and R5 detect and identify a probable difference in the amplitude of the 3.3V and 5V
output voltage limits.

Such difference in the voltage, could be as a result of differences in the output connected load or perhaps input voltage, are fed as a reference to the input pin of the TL431 shunt regulator.

The shunt regulator identifies these and compares the same with its in-built voltage reference level to trigger a feedback signal in the form of a current impulse trough U1 B proportional to the to the identified difference detected by R4 and R5.

Opto-coupler U1 immediately shuts down the control loop of the supply voltage by sending the feedback signal current to the output of the ENUV pin on the primary section of the circuit.

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Swagatam is an ardent electronic researcher, inventor, schematic/PCB designer, manufacturer, and an avid publisher. He is the founder of https://www.homemade-circuits.com/where visitors get the opportunity to read many of his innovative electronic circuit ideas, and also solve crucial circuit related problems through comment discussion.